Polyelectrolyte Brushes: Controlled Synthesis and Interface Dynamics

Kai Gao, Purdue University

Abstract

The incorporation of polyelectrolyte brushes as a surface modification technique has the potential to improve various technology by improving properties such as wettability, size selectivity, and microstructure control. Nanoscale polyelectrolyte (PE) brushes were developed through various synthetic methods, and the physics of PE brushes were characterized regarding their dynamic self-assembled nanostructures, stimuli-responsiveness and wettability. PE brushes were then used to functionalize nanoparticles and membranes by achieving hierarchal nanostructures. Novel assembly and suspension properties with brush functionalized nanoparticles and enhanced membrane performance were demonstrated. To investigate morphology evolution of weak polyelectrolytes, model polyacrylic acid (PAA) brushes were synthesized on silicon wafer substrates using a silane linker and Z-group RAFT method, which enables the controlled variation of brush characteristics. Atomic force microscopy (AFM) was used to characterize the brush morphology in dry state, which ranged from isolated globules to micelles to a continuous film featuring dimples. Surface morphology was primarily determined by total grafted polymer concentration with a critical polymer concentration between 4.5 and 6.5 × 10-23mol/nm2, where the polymer rich regions transition from micellar islands to a continuous film with dimples. However, even at the highest grafting densities and brush strand lengths used in this study, a flat homogenous film was not formed. Morphological consequences of the solvent and thermal history were shown to persist into the dry state. X-ray photoelectron spectroscopy (XPS) was used to quantify the degree of remaining salt after drying. It was found that a higher relative amount of salt was retained after drying in the lower grafting density samples. Applications of PE brush coatings were demonstrated by modifying membranes for controlling fouling and membrane transport. End-functionalized polyacrylic acid (PAA-silane) was synthesized with reversible addition-fragmentation chain-transfer (RAFT) polymerization and attached to both polysulfone ultrafiltration (UF) and polyamide reverse osmosis (RO) membranes through a non-impairing, one-step grafting-to approach in order to improve membrane surface wettability with minimal impact on membrane transport performance. After PAA grafting, composition and morphology changes on the membrane surface were characterized with Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). Static contact angle on PAA grafted membranes exhibited an increase in surface hydrophilicity and hence a potential enhancement in anti-fouling performance. The native contact angle on the polysulfone membrane systems was 86° and was reduced to 24° after modification, while the polyamide film contact angle decreased from 58° to 25°. The PAA layer endowed the porous UF membrane with dynamic control over the permeability and selectivity through the manipulation of the solution pH. The UF membrane with a 35nm average pore size displayed a 115% increase in flux when the contact solution was changed from pH 11 to pH 3. This effect was diminished to 70% and 32% as the average pore size decreased to 20nm and 10nm, respectively. Modified RO membranes displayed no reduction in membrane performance indicating that the underlying materials were unaffected by the modification environment or added polymer. Model polyamide and polysulfone surfaces were reacted with the PAA-silane inside a quartz crystal microbalance (QCM) to help inform the deposition behavior for the respective membrane chemistries. PE brushes were used to modify ultrafiltration (UF) membranes to facilitate the removal of phosphate ions during wastewater treatment. Polyacrylamide (PAAm) brushes were synthesized on polysulfone based UF membranes via an aqueous RAFT polymerization, which enabled the synthesis of controlled brush structures and was membrane material friendly. FTIR and XPS validated the attachment of PAAm brush modification through surface chemistry characterization. Improved phosphate rejection was observed on PAAm brush modified UF membranes with 10 and 35 nm pore size. There was a pH dependency with phosphate rejection on both neat and brush modified membranes, where phosphate rejection increases with increasing testing solution pH. As high as 70% phosphate could be rejected by one time filtration with PAAm brush modified PS-10 membrane at pH 10 condition. Normalized phosphate removal rate revealed that brush modified PS-35 membrane had the highest phosphate removal efficiency. From kinetic phosphate rejection test, brush modified demonstrated a long term steady phosphate removal performance.

Degree

Ph.D.

Advisors

Howarter, Purdue University.

Subject Area

Engineering|Materials science

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